Added a "Restart from Checkpoint" button

[InsightGit]

This PR adds a "Restart from Checkpoint" button to the pause menu in order to address issue #800, which requested for this sort of button to be added. The "Restart from Checkpoint" button will only appear underneath the "Restart Level" button if the player has more than 25 coins and has passed a valid checkpoint. If the player meets the conditions and triggers the button, it will subtract 10%(or 25) of the player's coins and restart the level at the past checkpoint.

Closes #800

[tobbi]

This looks good, I'll test it later today. 👍

[tobbi]

Hello @InsightGit Seems like this slipped through my fingers, sorry about that. It looks good, can you just resolve the conflicts and I'll merge this.

[tobbi]

There are two remaining things:

/home/travis/build/SuperTux/supertux/src/supertux/game_session.cpp: In constructor ‘GameSession::GameSession(const string&, Savegame&, Statistics*)’:
/home/travis/build/SuperTux/supertux/src/supertux/game_session.cpp:43:1: error: ‘GameSession::reset_checkpoint_button’ should be initialized in the member initialization list [-Werror=effc++]
 GameSession::GameSession(const std::string& levelfile_, Savegame& savegame, Statistics* statistics) :
 ^~~~~~~~~~~
/home/travis/build/SuperTux/supertux/src/supertux/game_session.cpp: In member function ‘virtual void GameSession::update(float)’:
/home/travis/build/SuperTux/supertux/src/supertux/game_session.cpp:410:35: error: ‘currentsector’ was not declared in this scope
     PlayerStatus *player_status = currentsector->player->get_status();
                                   ^~~~~~~~~~~~~
/home/travis/build/SuperTux/supertux/src/supertux/game_session.cpp:410:35: note: suggested alternative: ‘m_currentsector’
     PlayerStatus *player_status = currentsector->player->get_status();

[HybridDog]

maybe get_current_sector()->player->get_status();

[tobbi]

@HybridDog I don't suppose @InsightGit is interested in continuing this. Are you willing to take over?

[HybridDog]

Yes, I'll try to add the button.